How do fire investigators determine the cause of a blaze?

March 4, 2003

Dear Straight Dope:

I've always been a bit mystified by the apparent art/science of fire forensics--meaning the ability to walk into a burned-out house and determine where the fire started. I understand you could test for accelerants (e.g., someone doused the place with gasoline), but how do they work back to "a cigarette started it"? I would think by the time the fire is well underway in a room that everything is pretty much equally roasted. But fire marshals seem to be able to work their way back to the source. How is it done?

— Tab Julius, Ashland, NH

There's a common belief that the fire itself burns all the evidence into an unidentifiable mass of ash. But, in fact, that's not so. For a start, the fire department usually arrives in time to stop the fire from completely incinerating everything. Obviously, the more of the house or furnishings that are left, even in partly damaged condition, the easier it is to determine where the fire started.

The first clues to the "cause and origin" come from the firefighters. If the whole building is not yet engulfed, they can see the area where the flames are most intense, which is probably where the fire started and has been burning the longest. The firefighters note the color of the flames, the color and amount of smoke, the rate at which the fire spreads, and even the sounds the fire makes. These can be clues to what is burning and how it is burning--wood burns differently if it has been soaked with kerosene, for instance.

After the fire is extinguished, the investigators sift through the ashes, looking for clues. Who does the investigation varies by state and city, it could be the fire marshal or some combination of the fire and police departments. Sometimes insurance companies will have their own inspectors, as well. How do they determine the origin and cause? There can be a wide variety of indicators. Here are a few of the most common.

A very hot fire burns very quickly and moves very fast along its path, so that there can be sharp lines of demarcation between what is burned and what isn't. This is called the "char pattern" and it can be tracked backwards, like following footprints. For example, the char pattern around the door frame (even if the door itself was completely burned) would indicate which direction the fire came from--if the fire came into the room from the hallway, the part of the door jam outside the room (in the hallway) will be more heavily charred than the part that's inside.

Fire burns up, in a V-shaped pattern, so a fire that starts at an outlet against a wall (for example) leaves a char pattern that literally points to the origin. A very narrow V-shape might indicate a fire that was hotter than normal, such as one helped along by an accelerant (like kerosene or gasoline.) A U-shape could indicate that there was a "pool of origin" rather than a point of origin, such as might be caused by, say, a puddle of gasoline.

"Heat shadows" occur when heavy furniture shields part of a wall. Heat shadows can be triangulated, to help determine the origin point.

Fire burns upwards, so that ash may fall back down and smother the fire on the floor--for example, if a dropped cigarette starts a fire under the couch. The fabric couch burns faster than the wooden floor beneath, and the falling ash from the couch can extinguish the fire on the floor. When the ash is cleared away, the floor around the origin point may not be badly burned.

Light bulbs tend to melt toward the heat source, so the "direction of melt" can indicate the direction of the fire.

Since fire burns upwards, there can be a "chimney effect" where the fire ignites at a point, the superheated gases rise upward and form a fireball, which continues straight up to burn a hole in the ceiling. If the roof is not entirely burnt, and the fire investigator finds such a hole, the origin of the fire could be directly underneath.

Any or all of those can be clues to pinpoint the origin. The next task is to determine exactly how the fire started. Generally, the cause falls into one of three categories: natural, electrical, and chemical.

Natural causes are usually pretty easy to determine. There is plenty of evidence, and usually witnesses, when a house is struck by lightning or enveloped by a California brush fire, for instance.

Electrical problems are the most common cause of accidental fires. By examining a cord, or a fusebox, or an electric outlet, an expert can usually determine whether it was burned BY the fire, or was the SOURCE of the fire. Consider an electric outlet. The metal parts, for instance, probably don't burn but are charred; the plastic parts may melt from the heat; and the pattern of charring and melting can indicate whether the fire started inside the outlet or outside. The way the copper wiring melted or "splattered" could indicate an overheated circuit or an electrical short, respectively.

So, amid the ashes and ruin, the investigators search. Clearing away the ash often reveals unburnt clues. Again, there can be a wide range of indicators, but here are a few:

A fire with several distinct, independent origin points would arouse suspicion that the cause was not purely accidental.

The shattered or cracked glass of the windows can provide indications. A dark soot layer on the glass could indicate a slow, smoldering fire like a cigarette dropped on a couch. Clear glass with an abnormal pattern of cracking could imply a very hot fire, possibly due to an accelerant.

The depth or thickness of the ash can be indicative. For example, a thick layer of ash can mean an abnormally hot, fast-burning fire.

Fire burns upward, so a fire that burned downward will arouse curiosity. The most common reason for a fire to burn downward is the presence of a liquid accelerant poured on the floor before the fire started. The liquid soaked down into the floor, and then the fire followed it downward to the concrete slab.

If there are indications that a fire was, ah, assisted, the investigators may clear away all the ash and charred remains to expose the flooring--since fire rarely burns down, the floor beneath the fire is often not entirely burnt. Detective work might uncover purposely-made holes (to admit more oxygen), burn patterns that indicate spilling or pouring of fuel, and other evidence.

Although the ash may all look the same to the naked eye, spectrographic and chromatographic analysis can usually distinguish materials, and can almost always detect the presence of an accelerant. The presence of an accelerant does not, of course, necessarily mean arson--some people store flammable materials in unsafe places.

If the source of the fire is a piece of electrical equipment, an engineer specialist knows what a toaster or a gas stove looks like after it has been burned, and what it looks like if there has been sloppy installation, mechanical breakdown or tampering.

In short, fires leave tell-tale footprints for the experts to read.

There's a lot of money at stake, aside from the potential police issues. Insurance companies are unwilling to pay for fires that were set by the property owner, for instance, and are often willing to hire an investigator to prove it was so. Manufacturers of electrical appliances want to know if their product was the cause of an accidental fire. Government consumer product safety officials also take an interest in such incidents. It's rare that these investigators can't determine the cause and origin of a fire.

Many thanks to Fire Marshal Leonard Fabbri of the fire department of the City of Highland Park, Illinois, for providing fascinating information.

Fun reading:

Winslow, Don, California Fire and Life, Ballantine Books, 2001. This is a work of crime/mystery fiction, the protagonist being a fire investigator for a fictional insurance company, but it's a great read and chock-full of information about determining the source and cause of fires.

— Dex

Staff Reports are written by the Straight Dope Science Advisory Board, Cecil's online auxiliary. Though the SDSAB does its best, these columns are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed.